It lets you calculate what amount you need to charge so that after deducting taxes/commissions you get the exact amount you desire.
Now a days people are sellling thier services online and they need to pay different platform fee to different platforms like Fiverr charge 20% on every sale and steam community market charges 5% per sale. So if you're a seller and want to charge 50$ to your client you need to add that percentage amount in your invoice So that after all the deductions you'll get the amount you desired ($50). With This calculator you mention the amount you require and below mention the tax (platform % Charges). It;ll calculate the amount for you